1. Field of the Invention
This invention relates to a thermal head for making a stencil by thermally perforating a stencil material, and more particularly to an improvement of a thick film type thermal head.
2. Description of the Related Art
A thermal head generally comprises a heater element array formed by a plurality of heater elements arranged in a row extending in one direction (this direction is generally referred to as "the main scanning direction"), and when making a stencil, the thermal head is moved along a stencil material in a direction intersecting the main scanning direction (this direction is generally referred to as "the sub-scanning direction") while selectively energizing the heater elements, thereby thermally perforating the stencil material in an imagewise pattern. Such thermal heads are broadly divided by structure into a thin film type thermal head and a thick film type thermal head.
As shown in FIGS. 8 and 9, the thick film type thermal head conventionally comprises a ceramic substrate 85, a heat insulating layer 82 formed on the ceramic substrate 85, a plurality of comb-tooth electrodes 84 formed on the heat insulating layer 82 at predetermined intervals to extend in one direction in parallel to each other, and an electric heater strip 81 formed over the comb-tooth electrodes 84 to intersect the electrodes 84 in contact with the electrodes 84. The direction in which the electric heater strip 81 extends is the aforesaid main scanning direction and each of the parts between adjacent two electrodes 84 forms a heater element, whereby the aforesaid heater element array is formed. The main scanning direction is indicated at X in FIG. 8 and the aforesaid sub-scanning direction is indicated at Y in FIG. 8. The electric heater strip 81 is, for instance, of ruthenium oxide (RuO.sub.2), and is formed, for instance, by applying ruthenium oxide solution over the comb-tooth electrodes 84 by screen printing.
In order to improve recording density, the perforating pitch (that is, the distance by which the thermal head is moved in the sub-scanning direction at one time) should be as small as possible, and in order to reduce the perforating pitch, the width (the dimension as measured in the sub-scanning direction Y) of the heat generating area of the heater strip 81 (or each of the heater elements) should be as small as possible.
That is, if the width of the heat generating area of the heater strip 81 is larger than the perforating pitch, the perforations formed side by side in the sub-scanning direction Y will be merged with each other to form an elongated perforation as indicated at 102 in FIG. 10 (reference numeral 101 in FIG. 10 denotes a stencil material). When the perforations 102 are merged with each other into an elongated perforation, a large amount of ink flows out through the elongated perforation and an excessive amount of ink adheres to the printing paper, which can result in a phenomenon that the ink penetrates to the back side of the printing paper or the ink is seen from the back side of the printing paper. Accordingly, when the perforating pitch in the sub-scanning direction Y is to be reduced, it is necessary to reduce the width of the heat generating area of the electric heater strip 81 so that discrete perforations 102 can be formed in the sub-scanning direction as shown in FIG. 11.
In the conventional thick film type thermal head, the electric heater strip 81 generates heat over its entire width W, that is, each heat generating area or each heater element 87 has a length equal to the distance between the adjacent comb-tooth electrodes 84 and a width equal to the width W of the heater strip 81 as shown in FIG. 12. Accordingly, in order to reduce the width of the each heater element 87, it is necessary to form a narrower heater strip 81.
The heater strip 81 is generally formed by applying a paste-like mixture of, for instance, ruthenium oxide powder, glass powder and solvent by squeezing. In this case, the width of application of the paste-like mixture cannot be smaller than a mesh of the screen and the mesh of the screen cannot be smaller than the size of the particles in the paste-like mixture. As a result, it is difficult to form a narrower heater strip 81. If the particles contained in the paste-like mixture can be smaller in size, the mesh of the screen can be smaller, whereby a narrower heater strip 81 can be formed. However, the particle size is in proportion to the electric resistance of the heater strip 81 and accordingly, reduction in the particle size is limited. Further since the paste-like mixture has a certain viscosity, the mixture is kept in a limited area just after application thereof. However as the time lapses, the mixture flows and spreads outward. This phenomenon also makes it difficult to form a narrower heater strip 81.